# C++ attribute: maybe_unused (since C++17)

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Suppresses warnings on unused entities.

### Syntax

`[[maybe_unused]]` |  |

### Explanation

This attribute can appear in the declaration of the following entities:

- class/struct/union: struct [[maybe_unused]] S;
- typedef, including those declared by alias declaration: [[maybe_unused]] typedef S* PS;, using PS [[maybe_unused]] = S*;
- variable, including static data member: [[maybe_unused]] int x;
- non-static data member: union U { [[maybe_unused]] int n; };,
- function: [[maybe_unused]] void f();
- enumeration: enum [[maybe_unused]] E {};
- enumerator: enum { A [[maybe_unused]], B [[maybe_unused]] = 42 };
- structured binding: [[maybe_unused]] auto [a, b] = std::make_pair(42, 0.23);

For entities declared [[maybe_unused]], if the entities or their structured bindings are unused, the warning on unused entities issued by the compiler is suppressed.

For labels declared [[maybe_unused]], if they are unused, the warning on unused labels issued by the compiler is suppressed. | (since C++26)

### Example

Run this code

    #include <cassert>

    [[maybe_unused]] void f([[maybe_unused]] bool thing1,
                            [[maybe_unused]] bool thing2)
    {
        [[maybe_unused]] lb: // the label “lb” is not used, no warning
        [[maybe_unused]] bool b = thing1 && thing2;
        assert(b); // in release mode, assert is compiled out, and “b” is unused
                   // no warning because it is declared [[maybe_unused]]
    } // parameters “thing1” and “thing2” are not used, no warning

    int main() {}

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
CWG 2360 | C++17 | could not apply [[maybe_unused]] to structured bindings | allowed

### References

- C++23 standard (ISO/IEC 14882:2024):

  - 9.12.8 Maybe unused attribute [dcl.attr.unused]

- C++20 standard (ISO/IEC 14882:2020):

  - 9.12.7 Maybe unused attribute [dcl.attr.unused]

- C++17 standard (ISO/IEC 14882:2017):

  - 10.6.6 Maybe unused attribute [dcl.attr.unused]

### See also

C documentation for maybe_unused

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